1. Field of the Invention
The invention relates to an optical lighting system having a discharge tube, such as a xenon lamp and a metal halide lamp, as a light source and, more particularly, to an optical lighting system suitable for a video projector equipped with liquid crystal display devices.
2. Description of Related Art
There has been on the market what is called a video projector of the type equipped with liquid crystal display devices for forming images to be projected onto a screen. Such a video projector is equipped with a lighting system typically comprising a lighting system, an integrating device for providing an uniform quantitative distribution of light beams generated by the light source device and an optical system for lighting liquid crystal display panels by the uniform light beams. The light source device includes a discharge tube such as a xenon lamp and a metal halide lamp and a reflector, such as a parabolic reflector, an elliptical reflector and a hyperbolic reflector, for directing light beams to a desired direction.
A xenon lamp or a metal halide lamp has a spatial distribution characteristic such as shown in FIG. 5. As seen in FIG. 5, the majority of light beams generated by the discharge tube centralize within a spatial extent from an angle of 20 degrees to an angle of 140 degrees with respect to an electrode axis which is perpendicular to an optical axis of the light source device. In other words, the quantity of light beams which are parallel to and travel along the optical axis is not always large. In order to reflect light beams emanating from the discharge tube as much as possible, a large size of reflector is needed.
Even if a large sized reflector having a parabolic reflective surface as shown in FIG. 6 by way of example is employed, a lack of reflected light beams occurs near an optical axis of the light source device and the density of reflected light beams declines as the lateral distance from the optical axis becomes long, which always brings about aggravation of light condensing efficiency. Further, optical parts such as a condenser lens and an integrator associated with a large sized reflector in a light source device which must be large in size impose various constraint on designs of a compact lighting optical system and optical projection system for a video projector equipped with liquid crystal display plates.